1. Field of the Invention
This invention relates generally to axial flow fan and blade assemblies, and, more specifically, to large, industrial-type axial flow fans and improvements in the construction of blades therefor.
2. Background
It is generally preferable to use true tapered, twisted fan blades to achieve maximum air displacement efficiencies in large fan assemblies. It is also preferable to use lower cost fabricated light alloy fan blades in place of much more expensive molded fiberglass blades. But because of certain manufacturing complexities it has been practically impossible to reach suitable twist values in alloy fan blade airfoils without causing damage to the blade itself. One response to this limitation has been to add a tab to the trailing edge of an untwisted alloy fan blade in order to simulate the effects of a true twisted blade. Though this approach increases the effective displacement of air of an untwisted blade, a true twisted airfoil is still more efficient and desirable than the simulated effects of a modified trailing edge.
It is thus an object of this invention to provide an axial flow fan of the type described having blades fabricated from light alloy with a true twisted configuration to achieve maximum aerodynamic performance.
In connection with large, high performance fans it is also necessary to manage the stresses from aerodynamic loading generated in the airfoil skin. Heretofore fans of this type have utilized a relatively narrow cast, formed or fabricated transition piece from the fan hub to the airfoil section of the blade that limits the means of attachment to the airfoil skin and tends to concentrate stresses from the aerodynamic loading.
It is accordingly a further object of the invention to provide a axial fan having a transition from the fan hub to the airfoil section of the fan blade that not only allows for the use of a true twisted airfoil but also provides a mechanical joint between the airfoil skin and the transition piece which has a lower stress concentration pattern to better distribute aerodynamic loading forces.